The flowchart of study sample selection from National Health Insurance Research Database in Taiwan.

DM = Diabetes mellitus: ICD-9-CM 250; Osteoarthritis: ICD-9-CM 715; COX-2 inhibitor / Metformin: ≧ 90 days. Arthroscopic microsurgery was including synovectomy or/and capsulotomy (NHIRD order code 64054B-64057B), partial meniscectomy (NHIRD order code 64218B), and arthroscopic surgery (NHIRD order code 64243B-64244B). Joint replacement surgery was including osteosynthesis (NHIRD order code 64038B-64040B), total hip replacement (NHIRD order code 64162B-64168B), partial joint replacement (NHIRD order code 64169B-64170B), arthroplasty (NHIRD order code 64171B-64177B), arthrodesis (NHIRD order code 64178B-64183B), removal of prosthesis (NHIRD order code 64198B-64200B), revision replacement (64201B-64202B), and girdle stone procedure of hip (NHIRD order code 64203B).